Fix crash with -O3

Upon certain circumstances -O3 crashes if bb is None

Author: boriel

arch/zx48k/optimizer/basicblock.py

@@ -330,7 +330,7 @@ def update_goes_and_comes(self):
             bb = stack.pop(0)
             while True:
                 if bb is None:
-                    DummyBasicBlock(ALL_REGS, ALL_REGS)
+                    bb = DummyBasicBlock(ALL_REGS, ALL_REGS)
 
                 if bb in bbset:
                     break